1. Field
This invention relates to processes for selectively recovering a metal from a material containing other metal values in addition to the selected metal. In particular, the invention relates to integrated processes for selectively recovering a metal from a material containing such metal using selective, solvent extraction techniques.
2. State of the Art
Selective recovery of copper from aqueous solutions resulting from leaching copper-bearing minerals is well known. Organic, water-immiscible extracting agents are intimately contacted with the aqueous, copper-bearing solution resulting in a transfer of the copper values from the aqueous phase to the organic phase. The phases are allowed to separate in large settling tanks, and the copper recovered from the separated organic phase. The output of such processes has been limited by the size of the settling tanks, sufficient residency in the settling tanks being required to obtain satisfactory separation of the organic phase and the aqueous phase.
Applying a high-voltage electric field to an oil-water dispersion to coalesce water droplets has been used in the petroleum industry to remove small amounts of water from oil distillates and crude oil. Commercial application of electrical coalescence has generally been limited to dispersions containing only small amounts of water.
The electric field is customarily established by immersing a plurality of electrodes in the dispersion. As the water content of the dispersion increases, coalesced water particles have a tendency to form conducting bridges between the immersed electrodes. Several early patents (see, for example U.S. Pat. Nos. 1,116,299; 1,980,722; and 2,072,917) suggested utilizing alternating current or pulsating direct current to avoid the short circuiting of the electrodes caused by the formation of the conducting bridges. In U.S. Pat. No. 2,000,018, uni-directional, high voltage impulses, having durations of not greater than ten microseconds, with a time interval between impulses of at least 10 times the duration of the impulse, were required to be effective in inhibiting the formation of the conducting bridges.
It has also been suggested (see U.S. Pat. Nos. 3,074,870 and 3,567,619) to use a corona wind discharge to prevent the formation of conducting bridges. The isolation of the electrodes from the solution eliminates the conducting bridges; however, the corona wind discharge has been found to be rather ineffective in coalescing the water particles in the dispersion.
Recent developments in the petroleum field and related arts have been directed to improved apparatus and processes for applying potentials of very high gradients to hydrocarbon dispersions which contain very small amounts of water. U.S. Pat. Nos. 3,616,460 and 3,701,723 disclose systems using coaxially arranged electrodes, with the dispersion flowing longitudinally through the interelectrode space. U.S. Pat. No. 3,661,746 discloses a system for flowing the dispersion along a vertical flow axis and through an electric field which increases monitonically in potential in the direction of dispersion flow.
In a United Kingdom Pat. No. 909,485, processes are disclosed wherein an aqueous phase and an organic phase are first intimately mixed with each other, the dispersion or mixture being subsequently separated into its respective phases in a gravity settling tank. Improved settling rates were obtained by producing a high voltage electric field in the settling tank in the region of the emulsion band separating the aqueous phase from the organic phase.